Air core coaxial cables basically consist of an insulated signal conductor and a metallic outer shield separated from the inner conductor by a dielectric spacer. Air core twinaxial cables basically consist of two insulated signal conductors separated by dielectric spacers from a common metallic shield. In both designs, typically a core tube is included between each spacer and the surrounding metallic outer shield.
For many coaxial and twinaxial cable applications, achieving high signal propagation speed with less susceptibility to signal loss and distortion is a critical requirement. Examples of such applications include low-loss UHF/microwave interconnect cable, wireless telephony base station interconnect cable, semiconductor device testing equipment; instrumentation systems, computer networking; data communications, and broadcasting cable. For example, some coaxial cable designs for use in semiconductor testing require that the signal strength attenuation in dB per 100 ft. of cable be kept at or below 10 at frequencies of 6,000 MHz. Using larger conductors reduces cable attenuation; but to keep cable size small, low dielectric constant components are necessary.
High propagation speed coaxial and twinaxial cables of the prior art have used a variety of designs. In general, designers want to use as large an inner conductor diameter as possible since signal loss varies inversely with increasing conductor diameter. Larger inner conductor diameter sizes typically require larger volumes of dielectric spacer around the inner conductor to maintain the desired cable impedance. In order to maintain cable dimensions, this must be offset with increasingly lower overall dielectric constant values for the interior space separating the inner and outer conductors.
High-speed air core cable designs seek to maximize the air content between the inner and outer conductors, thus to realize the benefit of air as the ideal dielectric. Of course, air alone cannot supply structural stability; and therefore some relatively solid dielectric spacer must be included in an “air core” cable. These dielectric structures, while maximizing the air content, must meet a host of other requirements including: reliably uniform separation between the inner and outer conductors; resistance to deformation and crushing; heat resistance; ease of manufacture; and low cost. This combination of characteristics has proven difficult to realize commercially, as the following prior art illustrates.
U.S. Pat. No. 5,532,657 issued Jul. 2, 1996 discloses a coaxial cable in which an inner conductor and an outer conductor are separated by spirally-wrapped filament composed of low dielectric constant material such as polyolefins, polytetrafluoroethylene (PTFE) or mineral fibers. The filament may be a mono-filament or alternatively a dual-filament twisted pair. The filaments disclosed are circular in cross-section. The remaining space within the cable is air-filled, creating a dielectric area within the cable having lowered dielectric constant.
IBM Technical Disclosure Bulletin Vol. 32, No. 6A, November 1989 at p. 173-174, referred-to in U.S. Pat. No. 5,532,657, discloses a construction of coaxial cable where two individual filaments are spirally wrapped around a single center conductor in counter-directions and at different wrapping rates. The multiple crossings of the filaments are said to provide a stable symmetrical cross-section; and the interstices assure a large fraction of air dielectric in the cable. A similar construction using a twisted pair of filaments spirally wrapped around the center conductor is found in a coaxial cable product made by Temp-flex Inc. of So. Grafton, Mass. This twisted pair spacer is not in continuous contact with the center conductor, and therefore allows more air dielectric to contact the surface of the inner conductor.
The circular monofilaments have the drawback of placing circular cross-sectioned solid dielectric in close proximity to the inner conductor and thus increasing the effective dielectric constant of the cable. Further, while the twisted pair dielectric spacers of the prior art use less dielectric mass than a solid circular core monofilament—typically about 50% less mass—their manufacture requires providing two filaments instead of one, and having to use a complex twisting apparatus.
Foamed coaxial and twinaxial cable spacers are also found in the prior art. An early teaching in U.S. Pat. No. 2,890,263 issued Jun. 9, 1952 describes a UHF coaxial cable having an inner and outer corrugated conductor spaced apart in a first embodiment by a helically wrapped polyethylene or polystyrene strip selected for its low dielectric constant. The strip is shown as a solid core ellipsoid, which places dielectric mass close to the inner conductor. U.S. Pat. No. 2,890,263 also shows filling the interior space between inner and outer conductors entirely with foamed plastic material.
Greater durability and heat resistance for low-k spacer materials is provided by a process for introducing porosity in PTFE. U.S. Pat. No. 5,107,076 issued Apr. 21, 1992 shows a coax cable with a center conductor having tape-wrapped ribbons of porous or expanded PTFE fibers wrapped around it. Over this assembly is a tube or a tape-wrap of FEP; followed by an enclosing conductive metal layer. However, substantial dielectric mass is still positioned close to the center conductor in this design.
The need exists for both coaxial and twinaxial cables having propagation speeds greater than 1.22 Ns/ft.; and preferably of 1.15 Ns/ft. or less. In realizing such greater propagation speed, however, the cable designs should be attainable with a variety of spacer filaments either of the solid core design or of the foamed type, thus to provide a maximum of cable design flexibility. At the same time, the cost of manufacturing of these type cables must be as low as possible.